The crust of the Pilbara Craton is 28-33 km thick, and its boundary with the upper mantle dips south at slightly less than one degree. The southern edge of the craton is marked by a sharp increase in crustal thickness about 40 km south of the Sylvania Dome in the east and along the northern boundary of the Ashburton Trough in the west. Seismically, the crust has two layers, an upper one with P-wave velocity 6.0-6.1 km s^-1, and a lower of 6.4-6.6 km s^-1. The boundary between them is depressed beneath the deepest part of the Hamersley Basin, and rises along the southern part of the basin and craton, beneath a topographic ridge of higher grade metamorphic rocks. The velocities in the lower crustal layer are highest along the southern part of the Hamersley Basin, an area known to have been depressed during the formation and filling of the basin, and then uplifted. The crust of the northern Yilgarn Craton is at least 50 km thick and, seismically, has three layers. The uppermost layer has P-wave velocities of 6.1-6.2 km s^-1, the middle layer, at 10-16 km depth, 6.4 km s^-1, and the lowest, at 32 km depth, 6.7-7.0 km s^-1. A zone of crust, about 50 km wide, along the northern margin of the craton was extensively reworked during Proterozoic orogenesis, and is now characterised by thinner crust than the rest of the Yilgarn Craton, extensive granitoid emplacement, intense deformation and folding of the upper crustal rocks, and low gravity values. The Capricorn Orogen between the cratons is marked by high velocities in the lower crust, probably caused by dense mafic intrusions or a higher metamorphic grade. The seismic layering within the crust is most likely caused by increasing metamorphic grade with depth. The crust is probably of average acid to intermediate chemical composition, and has a low metamorphic grade at the surface. The velocities are attributed to metamorphic grade increasing to felsic granulite at 9-16 km depth, and garnet granulite at 32 km depth in the Yilgarn Craton.